﻿// Copyright 2004-2011 Castle Project - http://www.castleproject.org/
// 
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
// 
//     http://www.apache.org/licenses/LICENSE-2.0
// 
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#region 导入名称空间

using System;
using System.Reflection.Emit;

#endregion

namespace LightWork.DynamicProxy.Generators.Emitters.SimpleAST
{
    public class ConvertExpression : Expression
    {
        private readonly Expression right;
        private Type fromType;
        private Type target;

        public ConvertExpression(Type targetType, Expression right)
            : this(targetType, typeof(object), right)
        {
        }

        public ConvertExpression(Type targetType, Type fromType, Expression right)
        {
            target = targetType;
            this.fromType = fromType;
            this.right = right;
        }

        public override void Emit(IMemberEmitter member, ILGenerator gen)
        {
            right.Emit(member, gen);

            if (fromType == target)
                return;

            if (fromType.IsByRef)
                fromType = fromType.GetElementType();

            if (target.IsByRef)
                target = target.GetElementType();

            if (target.IsValueType)
            {
                if (fromType.IsValueType)
                    throw new NotImplementedException("Cannot convert between distinct value types");
                // Unbox conversion
                // Assumes fromType is a boxed value
                // if we can, we emit a box and ldind, otherwise, we will use unbox.any
                if (LdindOpCodesDictionary.Instance[target] != LdindOpCodesDictionary.EmptyOpCode)
                {
                    gen.Emit(OpCodes.Unbox, target);
                    OpCodeUtil.EmitLoadIndirectOpCodeForType(gen, target);
                }
                else
                {
                    gen.Emit(OpCodes.Unbox_Any, target);
                }
            }
            else
            {
                if (fromType.IsValueType)
                {
                    // Box conversion
                    gen.Emit(OpCodes.Box, fromType);
                    EmitCastIfNeeded(typeof(object), target, gen);
                }
                else
                {
                    // Possible down-cast
                    EmitCastIfNeeded(fromType, target, gen);
                }
            }
        }

        private static void EmitCastIfNeeded(Type from, Type target, ILGenerator gen)
        {
            if (target.IsGenericParameter)
                gen.Emit(OpCodes.Unbox_Any, target);
            else if (from.IsGenericParameter)
                gen.Emit(OpCodes.Box, from);
            else if (target.IsGenericType && target != from)
                gen.Emit(OpCodes.Castclass, target);
            else if (target.IsSubclassOf(from))
                gen.Emit(OpCodes.Castclass, target);
        }
    }
}